The present invention relates to tungsten heavy alloys, and more particularly to thermomechanical processing and manufacturing billets for kinetic energy penetrators, shaped charge liners, hyper-velocity projectiles, and others to enhance ballistic performance of warheads for breaking armor plates.
Penetrator requirements include high density and strength, moderate ductility, large length to diameter ratio geometry, and ability to display localized failure during impact to target (see Magkess, K. S., Jr., "High Strain Rate Deformation Behavior of Kinetic Energy Materials During Ballistic Impact," Mechanics of Materials, 18 (1994), 147-154). Depleted uranium penetrators satisfy these requirements but present safety and environmental problems. Therefore, tungsten heavy alloys (WHA) were developed. These alloys are two phase composites produced by powder metallurgy techniques in which the relatively small (10% or less) Ni base matrix phase acts as a binder and ductilizer for dense and strong tungsten grains. Many efforts have been devoted to improve mechanical properties of WHA by optimization of their composition and processing routes (see, Ravichandrau, G., "Influence of Processing on the High Strain Rate Behavior of Refractory Metal: A Review," Materials and Manufacturing Processes. V 9, No. 6 (1994); U.S. Pat. Nos. 4,990,195; 5,028,756; 5,306,364; and others). However, ballistic performance of WHA remains noticeably poorer than that of depleted uranium because tungsten heavy alloys do not exhibit localized flow and mushroom considerably during penetration of armor (see, Andrew, S. P., and Calgiuri, R. D., "A Review of Penetration Mechanisms and Dynamic Properties of Tungsten and Depleted Uranium Penetrators," Tungsten and Tungsten Alloys Recent advance, Eds. A Crowsen and E. Chen, Warrendale, PA, 1991). Also, attempts to induce localized deformation by replacing Ni with more adiabatic sensitive metals, development of the preferable texture into tungsten phase, or thermomechanical processing with moderate cold working (swaging and extrusion) were not fully successful.
Another approach is a structural modification of the binder phase by severe plastic deformation to achieve the highest level of strengthening and develop anisotropy for flow and fracture in the desired direction. Among the existing metalworking methods only the recently developed equal channel angular extrusion technique (ECAE) is the most suitable for that processing (see, Invention Certificate of the USSR No. 575892; Segal, V. M., et al., "Plastic Working of Metals by Simple Shear," Russian Metallurgy, 1 (1981), 115; Segal, V. M. "Simple Shear as a Metalworking Process for Advanced Materials Technology," First International Conference on Processing Materials for properties, Honolulu, 1993, 947-950). The known ECAE process and apparatus are shown in FIG. 1 (see, Segal, V. M., "Working of Metals by Simple Shear Deformation process," Proceedings Y International Aluminum Extrusion Technology Seminar, Chicago, 1992, 403-406). A well lubricated billet 1 is extruded by punch 2 through two meeting channels 3, 4 of a die 5. As channel cross-section areas are identical to that of the original billet, the billet is deformed by simple shear along the crossing plane of the channels. Following extrusion, the punch 2 returns to the initial position, and the worked billet 1 may be withdrawn from the channel 4. This operation can be repeated numerous times with changing a billet orientation between passes. Therefore ECAE offers opportunity to apply intensive and uniform strains to massive billets.
However, the known method and apparatus of equal channel angular extrusion are not without limitations. More particularly, their application for processing of tungsten heavy alloy penetrators presents a few problems.
First, the known methods of multistep equal channel angular extrusion provide the production of either strongly elongated grain structures or equiform grain structures. Elongated grain structures are not disposed for adiabatic localization and difficult in application to WHA as the dominant phase of the alloy is hard and non-deformable tungsten grains of near spherical shape. On the other hand, equiform grain structures produced by alteration of the shear direction along the same plane are suitable for adiabatic localization and processing WHA but exhibit strong anisotropy and asymmetrical flow along one set of slip lines during subsequent loading. This tendency increases with the increase of strain rate that will result in deviation of impact direction and ricochet of penetrators under interaction to a target. In addition, for ballistic performance penetrators should posses the special symmetry and anisotropy of planes under an optimal angle to the billet axis that can not be developed with known processing.
First, as penetrators are long rods of high strength material, friction inside the channel 3 (FIG. 1) significantly increases punch pressure. To eliminate this problem in the most advanced die design shown in FIGS. 1, 2, the first channel 3 has three movable walls fabricated into a slider 6. But in this case lubricant is forced to flow through small clearance "a" between the slider 6 end protrusion 7 covering the channel 3. That results in very poor friction condition and material sticking along the bottom wall of the second channel 4.
Also, an angle between adiabatic shear bands and a penetrator axis is of about 60.degree.. For that reason the protrusion 7 should have a sharp angle .phi. of insufficient strength, and may be destroyed during processing.